Environmental Engineering Reference
In-Depth Information
6.3 Instrument-based gas monitoring techniques
The last 15 years have seen major advances in the ability to make high-rate, near-
real-time volcanic-gas observations using compact, fully automated instruments.
The routine gas measurements now made with permanent monitoring networks
enable characterisation of magmatic degassing processes with unprecedented
temporal detail and resolution.
6.3.1 Fourier-transform infrared spectroscopy
The introduction of open-path Fourier-transform infrared (FTIR) spectrometers
in the mid 1990s (Mori
et al
.,
1993
;Francis
et al
.,
1998
) represented a major
breakthrough in volcanic-gas research that has promoted studies of the chemistry
of volcanic plumes. In particular, until the development of FTIR techniques,
open-vent (mafic) volcanoes were relatively unstudied compared with more
silicic arc volcanoes. A major advance occurred when incandescent rocks/
magma were used as a source of radiation, which allowed
first rapid (1-Hz),
real-time observations (
Figures 6.1b
,
6.3
) of the compositions of gases emitted
during lava-fountaining episodes (Allard
et al
.,
2005
), gas piston and lava
spattering events (Edmonds and Gerlach,
2007
), Strombolian explosions (Burton
et al
.,
2007
) and lava lake degassing (Oppenheimer
et al
.,
2009
). FTIR measure-
ments have demonstrated, for instance, a more CO
2
-rich chemistry of
fluids
released during mild (Hawaiian to Strombolian) explosive activity, relative
to quiescent gas emissions (
Figure 6.3
). This observation has important conse-
quences for understanding the mechanisms of gas generation, segregation and
loss that drive basaltic explosions (Allard
et al
.,
2005
;Burton
et al
.,
2007
).
FTIR has also contributed to volcano monitoring (Notsu and Mori,
2010
), with
the
first prototype fully autonomous FTIR systems now being tested (La Spina
et al
.,
2013
).
6.3.2 Multicomponent gas analyser system
A considerable amount of information on the chemical composition of volcanic
plumes has come from the Multi-GAS (multicomponent gas analyser system)
technique, which was
first applied to volcanoes in the mid 2000s (Aiuppa
et al
.,
2005
; Shinohara,
2005
). The Multi-GAS is based on assembling commercially
available infrared and electrochemical gas sensors into a single sensor kit
(
Figures 6.1c
,
6.4
). The reasonable cost, light weight and compact con
gura-
tion (
<
2 kg; 40
20
15 cm), robustness, and acquisition frequency up to
0.5 Hz has made the Multi-GAS ideal for analysing volcanic gas.
Figure 6.2
